This application is based upon and claims benefit of priority of Japanese Patent Application No. 2000-224089 filed on Jul. 25, 2000, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a meter for use in an automotive instrument panel, a pointer of the meter being driven by a stepping motor.
2. Description of Related Art
A stepping motor for driving a meter pointer includes a magnet rotor and field coils. The magnet rotor is usually magnetized to have plural poles, and therefore, the magnet rotor has plural stable positions in its rotation. If an overload is imposed on a rotational axis of the magnet rotor, the magnet rotor tends to take a wrong rotational position which is shifted from a right position to a neighboring stable position. In a speedometer for use in an automotive vehicle, this phenomenon causes an erroneous indication of a vehicle speed.
To alleviate this problem, in conventional meters, the pointer is over-driven toward a stopper positioned at a zero-position of a scale plate upon starting or terminating meter operation, and the pointer is forced to stop at the zero-position. Power supply to the stepping motor has to be stopped when the pointer hits the stopper. If the power supply to the stepping motor is not stopped when the pointer abuts the stopper, the pointer vibrates at the stopper position and generates noise in hitting the stopper repeatedly. Therefore, the halt of the stepping motor has to be detected. In conventional meters, the halt of the stepping motor is electrically detected based on a voltage induced in the field coils. That is, a voltage is induced in the field coils when the magnet rotor is rotating, while no voltage is induced when the magnet rotor is stopped.
However, it is not easy to detect the induced voltage in the field coils because the level of the induced voltage is too low. Usually, alternating pulse voltage is supplied to the field coils to drive the stepping motor, and its pulse width is made narrow to obtain a faster response of the stepping motor. The pulse width corresponds to a renewal time of the meter indication. Usually, the pulse width is set in a range of 0.5 to 1.5 milliseconds. The level of the induced voltage is not high enough to detect the motor halt based on the induced voltage.
The present invention has been made in view of the above-mentioned problem, and an object of the present invention is to provide an improved meter in which the pointer zero-position is properly adjusted. To surely detect the abutment of the pointer with the stopper, the level of the voltage induced in the filed coils is enhanced by increasing the pulse width of the driving voltage in the pointer zero-position adjustment mode.
A pointer of a meter such as a speedometer for use in an automotive instrument panel is disposed in front of a scale plate having an analog scale thereon. A stopper for stopping the pointer at its zero-position is provided on the scale plate. The pointer is driven by a stepping motor disposed behind the scale plate. The stepping motor is composed of a stator having field coils and a magnet rotor.
For adjusting the zero-position of the pointer, a driving pulse voltage having a wider pulse width (W2) is supplied to the field coils thereby to rotate the magnet rotor. When that driving voltage is at zero level, the voltage supply to one of the field coils is shut off, and a voltage (Vi) induced in that field coil is detected. The induced voltage (Vi) is compared with a predetermined threshold voltage (Vth).
If the induced voltage (Vi) is lower than the threshold voltage (Vth), it is determined that the pointer has stopped at the zero-position by abutting the stopper. At this point, the voltage supply to the field coils is discontinued. Thus, the zero-position of the pointer is accurately set. After the pointer zero-position adjustment is completed, a driving pulse voltage having a narrower pulse width (W1) is supplied to the field coils to operate the meter in an operating mode. Preferably, the pulse width (W2) is set to a level which is about two times the pulse width (W1). The pointer zero-position adjustment may be performed upon commencement of power supply to the meter or when the power supply is shut off.
According to the present invention, a higher induced voltage (Vi) is obtained because the driving pulse having a wider pulse width (W2) is supplied to the stepping motor in the pointer zero-position adjustment mode. Therefore, the zero-position is easily and accurately detected by comparing the induced voltage (Vi) with the threshold voltage (Vth), while avoiding pointer vibration otherwise caused at the zero-position. Further, the pointer response is quicker because the driving pulse having narrower pulse width (W1) is supplied to the stepping motor in the normal operation mode. Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiment described below with reference to the following drawings.